1. Field of the Invention
The present invention relates to electronic packaging. More particularly, the present invention relates to electronic packaging using flexible flip chip carriers with land grid array attachment arrangements for stiffening same.
2. Background and Related Art
There exists in the prior art a variety of ways of connecting integrated circuit chips to printed circuit boards (PCBs). One prior art approach is to connect the integrated circuit chip directly to the laminate PCB. However, one of the difficulties with such connections arises from the large difference in thermal coefficient of expansion (TCE) between the silicon chip and the PCB. This difference creates problems affecting reliability over a large number of thermal cycles. One approach to addressing the problems caused by differences in TCE between chip and PCB is to use some form of intermediate chip carrier. Such carriers offer several advantages including the fact that they may be either single chip carriers or multichip carriers. Typically, they are made of plastic, ceramic or flexible tape-like material. However, chip carriers made of plastic, ceramic or flexible material also have limitations in regard to overall reliability, cost and ease of assembly. Intermediate structures known as interposers, are also employed in a variety of design configurations to connect chips or chip carriers to laminate cards or PCBs. Typically, these interposers are designed to offer some additional form of compliance or flexibility in connecting chips or chip carriers to cards or PCBs so as to avoid stress, fracturing and the like caused by differences in TCE between chip or chip carrier and card or PCB.
Many advances are being made in organic packaging materials, such as, polytetrofluoroethylene (PTFE) based materials. Such materials, when used as flexible chip carriers, offer a low dielectric constant that improves electrical performance beyond that which is possible with standard FR-4 laminates, for example. Such materials also make possible improvements in wiring density. As chip density and machine speeds increase, advanced laminate materials for chip carriers become more important. In addition to reliance upon material advances, use of improved interconnect technologies, such as land grid array (LGA) interconnects, become more important.
The LGA interconnects rely on good surface contact between mating pads for reliable electrical connection. Accordingly, a constant and uniform compressive force must be maintained between the mating pads throughout the life of the electronic device in order to maintain reliable electrical interconnection. In addition, air must be excluded to prevent oxidation of the contact surfaces where noble metals are not used.
A primary requirement for LGA connectors is rigidity of the mating arrays. Thin packages, such as tape ball grid array (TBGA)-type packages and ones that use materials with low elastic moduli, such as PTFE or unfilled build-up layers, must have stiffeners attached in order to prevent excessive bending. Typical of such arrangements is the peripheral stiffener described by Tsukamoto in U.S. Pat. No. 5,841,194. However, even with peripheral stiffeners, force is transmitted through the package and produces a small amount of localized bending in the chip carrier substrate.
In addition to localized bending, plastic flow of the dielectric materials, especially when a laminate such as PTFE is used, occurs over time with an applied load, particularly at elevated temperatures. This acts to increase deformation. The compressive load on an LGA connection can cause relaxation of the laminate and a decrease in the contact force between mating pads over time thereby leading to intermittent or constant “open” connections.
Another problem that arises from this localized bending or deformation is the fracture of circuit lines or cracking of solder masks. Such fracture or cracking occurs because neither of these structures can deform plastically to the same degree as PTFE or an unfilled polymer, for example, can deform.